In an effort to increase the resolving power of X-ray apparatus in order to detect ever smaller details in X-ray photographs, for example extremely fine hairline cracks in turbine blades, there have been developed X-ray tubes of which the hot cathode is produced from ever finer wire and is formed as a needle point in order that the electron emitting surface--on the needle point--is made as small as possible. It was heretofore believed that only in this manner, corresponding to the rule of optics the smaller the light source the higher the resolution could a sharp X-ray picture be produced.
It succeeded considerably to increase the resolution power of X-ray apparatus in this manner but only at the price of a very limited electron emission and at the price of a greatly decreased life span of the hot cathode. The limited electron emission leads to the result that in the use of the X-ray apparatus in the medical field longer exposure time is required, thereby increasing the burden on the patient, while in use of the X-ray apparatus for material inspection, the penentration power is limited the inspection requires considerably more time and the possibility of inspecting moving objects is considerably limited. The greatly decreased life span of the hot cathode makes frequent changing necessary, a procedure in which, after changing the cathode, the X-ray tube must be again evacuated before it is ready for use. This is a time consuming procedure which very unfavorably influences the relation of useful time to down time.
Measures taken with respect to the target of the X-ray tube to solve the problem have led neither to avoiding the above mentioned disadvantages with respect to the cathode nor to an appreciable improvement of the intensity of the emitted X-rays, but rather to an early erosion of the target surface. With respect to the target, persons skilled in the art have strictly followed the rule of Heel that the target angle (angle between a perpendicular to the direction of incidence of the electron beam and the target surface) should be between 10.degree. and 40.degree. because intensity of the X-ray emission occurs at a target angle between 30.degree. and 35.degree.. As no improvement with respect to the target appeared possible, efforts to improve the intensity have been directed to the cathode of the X-ray tube but have ended in the above mentioned limitations.